1. Technical Field
This invention generally relates to a method of manufacturing electrodes and more particularly to a method of manufacturing electrodes in a vacuum arc remelting furnace. Specifically, the invention relates to a method of manufacturing titanium electrodes, which method includes the use of a reusable header for supporting the electrodes to be melted within the furnace.
2. Background Information
There is a need in industry for high metallurgical quality metals such as titanium, titanium alloys and superalloys. These products are utilized in the production of turbines for aircraft and ships and in various other industrial applications. Often, high quality titanium metal and its alloys are produced by a process known as vacuum arc remelting. In this process, an electrode, made of titanium material, is melted by a direct current arc into a water-cooled crucible or hearth under a vacuum. The electrode may be formed from scrap materials, titanium sponge compacts and bulk scrap pieces that are melted together by electron beam or plasma arc melting or are conventionally welded together. The electrode is then welded to a header or stinger that is connected to a ram. The welding of the electrode to the header is both time consuming and labor intensive and therefore adds to the production costs for the process. Once the electrode is welded to the header, a ram lowers the electrode into the crucible or hearth where it is melted by a direct current arc struck between the surface of the electrode and the crucible. Molten droplets of metal fall from the electrode onto the bottom plate of the crucible thereby forming a molten ingot pool. As the arc is struck between the electrode and the ingot pool, the depth of the ingot pool increases. The crucible or hearth is water-cooled and consequently the molten ingot pool gradually cools down and solidifies into an ingot. As the depth of the solidifying ingot increases, the ingot may either be slowly withdrawn from the crucible or will tend to gradually fill up the crucible. The process continues until the electrode is substantially consumed and an ingot of higher metallurgical quality has been formed. The newly formed ingot may be as long as 300 inches. The ingot is allowed removed from the crucible and is allowed to cool over a number of days. If a higher grade metal is required, the newly formed ingot is again welded to a header so that it may be used as a second electrode. The need to wait until the ingot has cooled and then to weld the second electrode to the header again adds to the production costs. The second electrode is remelted using the same process and a second ingot of still greater quality is produced. This cycle of forming an ingot, welding the ingot to the header so that it may be used as an electrode, and melting the electrode to form a new ingot of improved metallurgical quality is repeated until the desired metallurgical qualities are produced in the final ingot.
During production, the forming ingot may be contaminated by accidental arcing of the header. As the electrode is consumed, it is reduced in length. If, however, the length of the titanium electrode is reduced too much, accidental arcing of the header may cause some of the material from the header to melt and drop into the ingot pool. This tends to contaminate the titanium metal in the ingot pool and additionally causes damage to the header. In order to overcome this problem, it has been customary to stop the direct arcing of the electrode some distance from the weld between the header and the electrode. While this tends to resolve the problem of accidental contamination of the ingot and damaging the header, it also raises the cost of production. If, for example, the initial electrode is 300 inches in length and the direct arcing of the electrode must be ended around 3–5 inches from the weld of the electrode to the header, that 3–5 inches of electrode are waste material. The 3–5 inches of titanium may weigh around 500 lbs and the scrapping of this quantity of material from each phase of the melting process adds considerably to the costs of production. Furthermore, because accidental arcing and subsequent damage to the header may occur, there may be a need for the header to be periodically rebuilt or repaired. This again increases the cost of production.
There is therefore a need in the industry for a method of manufacturing electrodes in a more efficient and less expensive manner.